This invention relates to a method and apparatus for regulating the passage of fluids through chambers and other conduits. More specifically, this invention relates to a method and apparatus which accomplish such regulation by exploiting the relationship between the volume of the chamber and the volume of the fluid passed.
It will be appreciated by those skilled in the art that a fluid is a substance which alters its shape in response to applied force and which tends to flow or to conform to the outline of its container. Fluids include gases, liquids, supercritical fluids, plastic solids, and mixtures of solids and liquids capable of flow.
The passage of fluids through conduits provides the basis for numerous techniques in such fields as manufacturing, medicine, and scientific research. Conduits through which fluids pass may possess various shapes and dimensions. One shared characteristic of such conduits is the capacity to substantially contain and direct the flow of a fluid passed therethrough. It will be understood that a portion of a conduit having ascertainable dimensions may be known as a chamber or, alternatively, as a thimble or container.
One set of techniques involving the passage of fluids is the extraction of chemical compounds contained in matrices of solid support material. In many chemical extractions, a fluid which will dissolve a chemical compound of int.RTM.r.RTM.st is passed through a chamber or a portion thereof containing the matrix. A fluid employed in this manner is known in the art as an extraction fluid or extraction solvent; the dissolved species contained in an extraction solvent which has passed over the matrix is known as a solute. In most extractions, the fluid which has passed over the matrix is collected and the solute is isolated therefrom by various well-known techniques.
It is known, for example, to use supercritical and near-critical fluids in chemical extractions. As will be appreciated by those skilled in the art, slight changes in temperature and pressure in what is known as a fluid's critical region cause extremely large changes in the density of the fluid, and thus in its dissolving power. The supercritical region comprises all temperatures and pressures above which the distinction between liquids and gasses disappears. Supercritical fluids are thus a useful hybrid of gases and liquids which possess gas-like viscosities, liquid-like densities, and diffusivities greater than typical liquid solvents, commonly approaching values intermediate between those of typical gases and liquids. Compared to typical liquid solvents, the supercritical fluid properties of viscosity and diffusivity allow enhanced mass transport within complex matrices such as coal, plant or animal tissue, or packed beds.
Under conditions where the pressure exerted upon the fluid is less than the critical point pressure and the temperature is such that the actual pressure of the fluid is greater than about seven tenths of the critical pressure (i.e., at reduced pressures greater than about 0.6) and the density is greater than about two tenths of the critical density (i.e., at reduced densities greater than about 0.2), the resulting gas is dense enough to provide enhanced solvation and vaporization of solutes into th gas. Compressible liquids near the critical point have liquid-like densities and diffusivities and viscosities similar to supercritical fluids. Thus, due to enhanced mass transport properties, such compressible liquids are more attractive as solvents than typical, virtually-incompressible liquids.
Accordingly it is often desired in extractions and other applications that the passage of a fluid be carefully controlled or regulated. It will be appreciated by those skilled in the art that there exist myriad methods for effecting such control. It is quite common, for example, to terminate the passage of a fluid after a pre-determined mass of the fluid has been passed over a matrix bearing a chemical compound. It is also known to terminate such passage after a pre-determined volume of an incompressible fluid has been passed. In another method, the quantity of extracted solute is monitored and the passage of the fluid is terminated after a certain amount of that solute is obtained.
Unfortunately, however, many such methods for regulating the passage of fluids have significant disadvantages. For example, continued monitoring of the amount of solute extracted can be burdensome. Also, the amount of solute extracted from a matrix may not necessarily correspond to the mass or volume of extraction solvent employed. This is particularly true where mass or volume are measured by reference to mass or volumetric flow rates determined in a pumping region which is held at constant or controlled conditions different from the variable conditions of a chamber downstream from the pumping region. Thus, regulation of fluid passage in such extractions by reference solely to the mass flow or pump volumes of solvent employed will frequently be inefficient, largely failing to take into account that the residence time within the chamber varies according to the pressure and temperature settings of the chamber. This is true even where the mass flow rate of the system or the volume flow rate at the pump remain constant. Frequently, neither the extent of contact of the material with fresh extracting fluid nor possible mechanisms by which solvents diffuse through matrices comprising extractable material are properly taken into account.
Thus, it would be of great advantage to regulate the passage of fluids in chemical extractions and other applications by techniques more efficient and less labor-intensive than those presently known in the art, perhaps by techniques which account for the relationship between either the size of the extraction sample or the volume of the chamber and the volume of extraction fluid employed.